The present invention relates to a reflector for use in solar radiation collecting systems.
In general, solar radiation can be used, even with good exergetic efficiency, in two particular ways:
1. Solar energy can be converted immediately into electrienergy by means of photovoltaic cells or into mechanical or refrigeration energy, e.g., by means of thermodynamic engines; PA1 2. Solar radiation can be stored over a short or a longer period of time either in form of heat at a temperature above that of ambience, by using entropy-changing matter, or through a chemical reaction in the form of chemical energy, such as by photosynthesis of high-energy compounds in a photochemical reactor.
The conditions for making use of either one of the above-mentioned techniques will depend, first, upon the properties of the technical systems to absorb highly concentrated radiation; second, upon the availability of reflectors with a high concentration; and, third, upon the reduction of interference by adverse weather conditions both on the operation of the radiation-absorbing system proper and the concentration provided by the reflector.
Concentration of solar energy irradiated from the sun by means of concentrating reflectors, having a concentration coefficient or rate C (1.ltoreq.C&lt;1000 is already proven) has the consequence that the absorbing system must, in principle, not differ in its dimensions from that of the focus. With an increasing concentration rate C, the power density within the absorbing system will increase while the quantity of material used decreases. In addition, the heat losses of the absorbing systems are, due to back-radiation and convection, drastically reduced, when compared with absorbing systems using concentrated radiation, because of their much larger exposed surfaces; as a result, the operation temperatures obtainable with concentration of radiation are much higher.
The use of concentrated radiation, however, is feasible only if the absorbing system and its reflector are integrated into a solar-energy-converter concentrator unit and track the sun. A unit of this kind has its, necessarily large and sensitive, surface oriented normal to the radiation and is usually exposed to great aerodynamic forces. Any relevant airflow will exert rather large forces upon the unit itself, mainly, on its reflector, the tracking device, and the anchoring. Dust, larger particles, and rain will destroy the reflecting surface. At more northern latitudes, frequent cloud covers reduce the average operation periods and, thereby, the economy of the system as a whole.
The design and construction of concentrating reflectors suitable for absorbing systems, either for conversion or storage of radiation and to be integrated in such a system, will ultimately determine the feasibility of large-scale,, technical implementation of solar energy systems in parallel with and independently from further developments of the advanced systems themselves. A number of proposals have been made so far for the development of concentrating reflectors at low costs and having an extended life span. In addition, it has been considered to lift the reflectors and absorber units from the ground by filling a carrier with a gas of lower density than ambient air. This way, one can attempt to exclude interference of the operation or radiation absorption by weather and climate conditions on the ground (see, for example, U.S. Pat. No. 4,002,158).
The problem at hand should be discussed in some detail. A "classical" parabolic dish reflector is, for example, made from sheats and covered on the inside with a glass mirror segment. The relative aerodynamic friction force is presumed normalized at .sigma..F.p=1. .sigma. is the aerodynamic friction factor, F is the surface normal to wind velocity vector, p is the dynamic pressure head. Assuming that this kind of reflector is unprotected, it must withstand great aerodynamic forces which is reflected in a very robust design. Such a reflector could be installed within a transparent radom in order to protect it from wind forces and to facilitate construction of both reflector and tracking device (an example is the BOEING System). In spite of the fact that the radom has a much larger surface projected normal to the wind vector than the unprotected mirror, the relative aerodynamic friction force is .sigma..F.p=0.67 due to a more favorable friction factor .sigma..
In the case of a bubble- or radom-protected reflector, the forces exerted upon the reflector proper are smaller. For this reason, a light-weight construction principle in regard to the reflector can be applied. The radome provides a thin, transparent, protecting envelope; and the reflector could be made from a thin and properly formed foil which has reflecting properties due, for instance, by sputtering it with aluminum. The foil mentioned above can be brought into a well-defined paraboloid shape generated under tensile load and by a pressure difference between its inside and outside; for this, it is necessary to anchor the foil on its border to a rigid, supporting cylinder, to close the cylinder at its other end, and to slighty diminish the pressure of the enclosed air. The cylinder can also be replaced by a cylindrical, frame-like structure (an example: the KLEINWACHTERBOMIN system).
As mentioned before, the economy of a concentrating solar energy converter unit does not only depend upon the amount of material used to produce the reflector, but also on the long duration of useful operation. It is, therefore, appropriate to place both the reflector and the absorbing system above the main cloud layers. This is possible only if the unit as a whole is buoyant (see U.S. Pat. Nos. 4,002,158 and 4,127,453). Due to the fact that the pressure differences necessary to shape the foils are quite low, it is, indeed, possible to generate large aerostatic buoyancy forces by, for instance, filling the interior of appropriate compartments with hydrogen; large, hydrogen-filled cushions can be added in order to reduce the supporting, rigid structure and to increase overall the interior gas volume. By this means, the surface opposed to the wind will, however, increase; the relative friction force .sigma..F.p=1.96.
Another alternative for obtaining aerostatic buoyancy employs the radom in conjunction with a light-weight structure. A properly shaped, reflecting foil devides a spherical radom in two compartments with different gas pressures in order to generate the curvature of the foil. The relative friction force is decreased and .sigma..F.p=0.49. The advantage of this alternative can, in addition, be seen in the possibility of avoiding hydrogen filling and replacing the hydrogen with hot air.